Energetic and rate effects on methanogenesis of ethanol and propionate in perturbed CSTRs

Energetic and reaction‐rate interactions between hydrogenic (hydrogen‐producing) and hydrogenotrophic (hydrogen‐consuming) bacteria were investigated in five perturbation experiments performed on steady‐state, mixed‐culture methanogenic CSTRs receiving ethanol, propionate, or both hydrogenic substrates. When a large quantity of propionate was suddenly added to a propionatefed CSTR, P   H   2increased to 10 −4 atm and propionate oxidation remained energetically favorable. When ethanol was added to a CSTR receiving ethanol, P   H   2rose to 6.3 × 10 −3 atm within 5 h. In both perturbations, P   H   2remained at levels such that oxidation of the hydrogenic substrate remained energetically favorable throughout the transient. Sudden increase in ethanol concentration in the ethanol‐ and propionate‐fed CSTR resulted in an increase in P   H   2such that propionate oxidation became energetically unfavorable and was blocked. Propionate utilization resumed when the added ethanol was depleted and P   H   2returned to its previous steady‐state levels. Ethanol perturbation of ethanol‐ and propionate‐fed CSTRs led to the formation of reduced products, including n ‐propanol and four‐through seven‐carbon n ‐carboxylic acids, when P   H   2was elevated; these products disappeared after P   H   2returned to previous, steady‐state levels. The transformations were consistent with reaction energetics. Reduced product formation may have been a sink for reducing equivalents, as an alternative to oxidation for propionate utilization, as indicated by an electron equivalents balance over the time course of experiments.